A typical conventional mirror bias system for a differential amplifier includes a differential amplifier responsive to input signals and a current mirror that includes a tail current device on the output leg and a current reference device on the input leg. However, when the common mode voltage applied at the inputs of the differential amplifier changes or a differential change in voltage is applied to the inputs, the voltage on the drain of the tail current device changes and will not match the voltage on the drain of the current reference device. The result is that the current in the tail current device on the output leg no longer matches the current in the current reference device on the input leg due to the finite output resistances associated with the tail current device and the current reference device.
An improved prior art mirror bias system for a differential pair similarly includes a differential amplifier and a current mirror but adds a cascode device to the input leg connected to the current reference device. The gate voltage of the cascode device is set to a fixed voltage, VBIAS, that fixes the voltage on the drain of the current reference device to be equal to VBIAS−VGS. The cascode device is typically sized to run at the same current density as the transconductance components of the differential amplifier. When VBIAS is set equal to the common mode voltage of the differential amplifier the voltage on the drain of the current reference device approximately matches the voltage on the drain of the tail current device and the effect of mismatched current due to the finite output impedance of the current reference device and the tail current device is cancelled. The drawback of such a design is that the cascode device is designed to be biased for a predetermined fixed input common mode voltage. The design does not account for changes in the input common mode voltage or a differential change in voltage applied to the inputs of the differential amplifier.